This invention relates to an amplifier type solid state imaging apparatus formed on a silicon substrate and, more particularly, it relates to a solid state imaging device wherein each unit cell has a photodiode processed for device separation by means of a silicon oxide film formed by oxidizing the silicon substrate. The present invention also relates to a solid state imaging apparatus wherein the semiconductor substrate of the apparatus has for each unit cell a region located at a position deeper than the depletion layer region operating as a photodiode, in which region the impurity concentration of the semiconductor slowly increases as a function of the depth in the substrate and part of the signal charges generated in the semiconductor substrate are collected by a signal storage to provide a high dynamic range. The present invention further provides a method of manufacturing such a solid state imaging apparatus and a video system realized by using such a solid state imaging apparatus.
Solid state imaging apparatuses comprising an amplifier type sensor have been developed in recent years. Such apparatuses are featured by detecting optical signals by means of a photoelectric converter/storage and amplifying them in the vicinity of the photoelectric converter/storage.
An amplifier type MOS sensor typically comprises in each unit pixels or unit cell thereof a photodiode and amplifying means including an amplifier transistor for amplifying the signal charges photoelectrically converted and collected by the photodiode in the silicon substrate.
FIG. 1 of the accompanying drawings schematically illustrates in cross section part of a unit cell of a known amplifier type MOS sensor. As seen from FIG. 1, an n-type layer region 12 that constitutes a photodiode with a silicon substrate (p-type layer region) 10 is formed in an oxide film for device separation in a self-aligning manner. A device separating region 16 arranged on a p.sup.+ -layer 14 is a silicon oxide film formed by oxidizing part of the silicon substrate 10, which silicon oxide film is normally referred to as LOCOS (LOCal Oxidation of Silicon). Reference numerals 18 and 20 in FIG. 1 denote respectively a contact region and a wiring layer connected to the contact region 18, whereas reference numerals 22 and 24 denote respectively the gate of a read-out transistor and a planarizing layer.
The silicon substrate 10 is apt to become defective at and near the corresponding end of the LOCOS region 16 due to the stress generated during the local oxidation. The defect, if any, by turn gives rise to an electric current that appear as leak current of the photodiode.
Now, this problem will be discussed by referring to FIG. 2 of the accompanying drawing.
FIG. 2 is an enlarged cross sectional view showing the boundary of the photodiode and the LOCOS region of FIG. 1. As shown in FIG. 2, a depletion region 26 is formed around the n-type layer region 12 and a depleted region with a large number of defects (multi-defect region) 28 is formed in a lower boundary area of the LOCOS region 16 located adjacent to the n-type layer region 12. Thus, a large number of electron/hole pairs will be generated by heat via the defect levels in the silicon band gap in the multi-defect regions. Then, electrons can transfer into the photodiode to appear as leak current of the photodiode, which leak current can reduce the sensitivity or the S/N ratio of the solid state imaging apparatus.
Thus, since a photodiode and a photodiode are formed in a self-aligning manner in known solid state imaging apparatus, they are accompanied by the problem of leak current on the part of the photodiode generated due to the defect at and near the corresponding end of the LOCOS region 16.